Process for the pickling of metals



252. COMPOSITIONS,

Patented Aug. 12, 1930 UNITED STATES Examiner ERNEST CHAPMAN AND ARTHURHILL, BLACKLEY, MANCHESTER, ENGLAND, AS- SIGNOBS T0 IMPERIAL CHEMICALINDUSTRIES LIMITED, OF LONDON, ENGLAND PROCESS FOR THE PICKLING OFMETALS 80 Drawing. Application filed April 12, 1929, Serial No. 354,696,and in Great Britain April 14, 1928.

It is known that thioureas and substitution products of the type inwhich R represents hydrogen, alkyl, aralkyl or aryl radicles act asinhibitors against the dissolution of metallic iron in pickling andsimilar processes. This however is an empirically ascertained fact, and,in the present state of knowledge, it is impossible to state the causeof this inhibitory effect or to predict what bodies will and what willnot be effective inhibitors in the pickling process. By efiectiveinhibitor we understand a body which while not retarding the cleansingaction of the metal surface, i. e. the removal of metallic oxide or rustby acid, prevents or greatly hinders attack of the metal itself. It isalso known that organic compounds containing a mercapto group areeffective inhibitors in processes for the pickling of metal. This isdisclosed in the British Patent No. 298,390 and the U. S. patent toChamberlain In the Chamberlain patent there is disclosed under thebroad'class of mercapto compounds the species of compounds known asmercapto thiazoles.

We have discovered that this inhibiting function is not confined to themercapto thiazoles but the general class gf bodies known as th i azoledeg ivativesf'af"very excellent inhibitor s against attack of metalliciron or steel by acids and consequently of great value as additions topickling and similar acid of cleansing baths.

By thiazole derivatives we mean those 7 40 bodies containing thefollowing structural arrangement wherein X represents hydrogen, a groupor radicle of simple or complex structure such for instance as CH --C HSH,

Rl\ H or the following group C-R-NH:

asher'eiiiafteimoie fully set forth. In the Chamberlain patent mentionedabove there is specifically disclosed mercapto benzthiazole used as aninhibitor, but we have'now Toii'nd that this useful property appears tobe possessed by tl 1iazoles,benathiazoles, naphthg thiazoles andanthrathiazoles in general. As an example of typical inhibiting actionexercised by dehydrothi9;p ;t0lnidine-or the correspondingprimuline basewe mention that i 0.2 per cent by weight (reckoned on the Wei ht ofdilute acid used) of either of these su stances be added to 25 per centsulphuric acid (for cold pickling) or 5 per cent sulphuric acid (for hotpickling), the acid is found not to attack the metal to any appreciableextent during pickling, whilst without thebaiddition the metal is etchedvery considera We have found that the product from the interaction ofsulphur and o-toluidine under conditions sim"1lar-to'those"6btaininginthe manufacture of dehydrothio-p-toluidine or primuline can be convertedinto a form suitable for use as an inhibitor by simple solu tion in 7 0per cent sulphuric acid.

' For example, we may heat together 7 parts of o-toluidine and 2 partsof sulphur for 6 hours at 185 C. and then for 18 hours at 210215 C.Excess of o-toluidine is removed by distillation, preferably in steam orDu v v o-4,

acid of other strengths.

under reduced pressure. The tarry product, from which water is decantedif necessary, is dissolved in 9 times its weight of per cent sulphuricacid, or in suitable quantities of The solution so obtained is added insmall proportion to the pickling acid: e. g., 1 lb. of the solution issufficient for 4000 lbs. of 15 per cent hydrochloric pickling acid. Thisproportion may be varied to suit special conditions.

e compositions of the body obtained as above described from otolnidinenay be represented by the conventional formula fordehydrothio-o-toluidine.

but it also contains more highly sulphurized bodies of the primulinetype, the formula for which is not definitely known.

' p-toluidine or suitable xylidines, i. e. such as contain a freeortho-position adjacent to the amino group, may be used in place ofo-toluidine.

If it is preferred to use an isolated thiazole derivative instead of asolution in sulphuric acid, it ma be diluted with salts such as sodiumsulp ate. We have discovered that the inhibiting function is possessedby a general class of compounds containing a certain, structuralarrangement, this structural arrangement being different from that ofthe general classes of compounds heretofore known as inhibitors. In theChamberlain patent hereinbefore mentioned the inhibiting action wasattributed to the presence of the mercapto group and it was not knownthat the thiazole groupin of certain specific compounds classified un erthe broad class of mercapto compounds was involved in the inhibitingaction. We have found that compounds possessing the thiazole structurefunction as inhibitors irrespective of whether or not the mercapto groupis present. In this application we claim the compounds possessing thisinhibiting action which heretofore have not been utilized in processesof pickling of metals; that is thiazole derivatives containing thefollowing general structural arrangement I bR1 -o-N wherein R representshydrogen,

/R CH;,-C|Hs,N .,BNH: I

or the following grouping B h-Q e wherein R represents a phenyl residuewhich may or may not be further substituted with alkyl groups. Thepresent invention is more particularly directed to certain compoundsunder this general class, namely, the dehydrothiotoluidine bodies.

What we claim-and desire to secure by Let-- ters Patent is:

1. In a process of pickling metal, the step which comprises adding tothe pickling acid bath an inhibitor to diminish the attack of the acidupon the metal without hindering the removal of scale or dust, the saidinhibitor being a thiazole derivative containing the followingstructural arrangement wherein R represents hydrogen,

. CE4, l i, N I RNH:

or the following grouping R\N%C R NH:

wherein R represents a phenyl residue which may be further substitutedwith alkyl groups.

2. A process according to claim 1 in which the thiazole derivative usedis a dehydrothiotoluidine body.

3. A process according to claim 1 in which the thiazole derivative usedis dehydrothiotoluidine.

4. In a process of pickling metals, the step which comprises adding tothe pickling acid bath a solution of a dehydrothiotoluidine body insulphuric acid.

5. In an inhibitor for metal pickling baths, a new composition of mattercomprising a strong aqueous solution of sulphuric acid and the reactionproducts of sulphur and orthotoluidine dissolved in said strongsolution.

In testimony whereof we aflix our signatures.

ERNEST CHAPMAN. ARTHUR HILL.

